We have a water tank that's just 6.5 - 7 ft above ground level. The length of the pipe from water tank to sink tap would be around 25 ft. Output from water tank is an 1.5 inch pipe which then reduced to 1 inch pipe after 5 ft. Then the remaining 20 ft (1 inch pipe) is connected to sink tap which then finally reduced to 0.5 inch as per sink tap size. Sink tap is 3 ft above ground level. Due to the lower tank height and thus lesser gravitational pressure (???), the flow rate (also pressure ?) in the sink tap is not optimal (or I could say it's bit below usual flow rate). So plumber is considering following strategies to increase the flow rate (pressure ?) at the sink tap.

  • Increase the tank height further 3 ft (10 ft above ground level)

  • Starting 10 ft length would be covered by 1.5 inch pipe

  • Reduce from 1.5 inch to 1.25 inch after 10 ft

  • Next 10 ft length would be covered by 1.25 inch pipe

  • Reduce from 1.25 inch to 1 inch after that

  • Remaining 5 ft length would be covered by 1 inch pipe

  • Finally the 1 inch pipe is reduced to 0.5 inch at the sink tap connection

Obviously increasing the tank height would increase the flow rate (due to increased pressure), but does gradually reducing the pipe size (3 times) along the way increases the flow rate (pressure ?) at the end (sink tap) than what it was before?

  • Sure, you can do all that. Or, you can learn a little bit about how hydraulics work. I believe you’re interested in concepts like “levels” and “head”. From there a number of flow calculators should tell you if you have enough pipe (I’m pretty sure you do). Commented Jul 21, 2020 at 18:15
  • If you decrease the area for a given flow the restriction can cause a pressure increase. But if the flow is 0 changing the diameter will do nothing it will have the static pressure value. Also if you have restrictions limiting the flow like the size of the pump reducing the diameter may do very little to the pressure until the pipe is reduced to the point the flow is reduced
    – Ed Beal
    Commented Jul 22, 2020 at 0:17

2 Answers 2


Reducing the pipe diameter progressively would not increase the pressure nor flow rate at the tap. The idea is based on a nozzle at the end of a hose giving rise to a high speed jet of water. The nozzle actually reduces the flow rate (volume/time) through the hose and so increases the pressure just before the nozzle. This gives a higher velocity stream but lower flow rate. I assume what you want is a higher flow rate from your faucet.

Notice: I think I am right on my claim in the above, but frankly I am not sure. See edit below*.

You would get a higher flow rate from the faucet if you remove (unscrew) the aerator on the end of the faucet. This is a flow restrictor. Increasing the head would also increase pressure and flow rate.


The most basic formula for laminar (smooth) flow of water in a circular cross section pipe is Poiseuille's equation. To paraphrase, this states that the volume flow rate for an incompressible fluid is equal to the applied pressure times the cross sectional area of the pipe divided by (the length of the pipe times viscosity times 8). This means that progressively decreasing the diameter of the pipe would decrease flow and not increase it.

  • My idea was that maybe someone who was versed in fluid dynamics would answer. I put out my answer to influence the OP to not hastily spend time and money on this idea of progressively reducing the diameter of the supply piping in the hope that this would increase flow. I am 99 percent sure that this would not help, but fluid dynamics is very tricky. Commented Jul 23, 2020 at 11:24
  • OK @psaxton I did do some researching as you wished and edited my answer by invoking Poiseuille's equation. Commented Jul 23, 2020 at 22:59

Simple answer : any reduction in diameter at any location will potentially reduce flow and/or pressure, depending on the volume of fluid. With a low volume ,it will make no difference. With a high ( potential) volume ,it will be reduced.

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